1. Field of the Invention
The present invention relates to an ink ribbon cassette for an impact printer such as a serial wire-dot printer.
2. Description of the Related Art
In recent years, in contrast to the fact that more compact ink ribbon cassettes are required because the impact printers are becoming more compact, a prolongation of the service life of the ink ribbon cassette is also demanded. To respond to such a requirement, for example, an ink ribbon cassette disclosed in Japanese Unexamined Utility Model Publication No. 62-128850 and in Japanese Unexamined Utility Model Publication No. 63-51756 uses an endless ink ribbon. In this type of ink ribbon cassette, an ink ribbon storage unit, which stores the ink ribbon in a zigzag manner, is provided and an ink ribbon passage going from a first side of this ink ribbon storage unit toward a second side thereof is formed. Where the ink ribbon is made to run from the first side of the ink ribbon storage unit through the ink ribbon passage to the second side thereof, a ribbon feed roller, with a pressure roller elastically pressed against the ribbon feed roller, is provided on the second side of the ink ribbon storage unit. In particular, the ribbon feed roller is arranged in such a manner that the ink ribbon is wound around the ribbon feed roller over substantially a half circumference thereof, while the pressure roller is elastically pressed against the ink ribbon. When such an ink ribbon cassette is mounted on a carriage of the printer, a drive shaft projected from the carriage is received and engaged in a hole of the ribbon feed roller. During the printing operation of the printer, the ribbon feed roller is driven to rotate in the predetermined direction by the drive shaft, whereby the ink ribbon is sequentially drawn from one side of the ink ribbon storage unit and subsequently passes through the ink ribbon passage and then is drawn to the other side of the ink ribbon storage unit and stored again there. With the arrangement as mentioned above, the ink ribbon can be used over a long period until the service life thereof is ended, that is, until the ink ribbon is physically broken.
Moreover, to save natural resource, it is also a general practice to make the replacement of the ink ribbon in the ink ribbon cassette possible. In this case, the ink ribbon cassette per se can be used semipermanently. Such an ink ribbon cassette with a replaceable ink ribbon is constituted so that, at the time of replacement of the ink ribbon, the pressure roller is locked at a withdrawn position at a distance from the ribbon feed roller, whereby the replacement of the ink ribbon is easily carried out by the user.
However, the user is liable to make a mistake and may mount the ink ribbon cassette on the printer while the pressure roller is locked at the withdrawn position after the replacement of the ink ribbon. At this time, the ink ribbon cannot run through the ink ribbon passage, and therefore during the operation of the printer, the printing impact is concentrated at only a specific portion of the ink ribbon. Even if the user becomes aware of such an error and makes adjustments to allow the ink ribbon to run, such a portion is weakened in comparison with the other portions, and thus a problem that the service life of the ink ribbon is shortened.
On the other hand, an endless ink ribbon is formed by bonding the two ends of a long length of an ink ribbon by ultrasonic bonding. When this bonded portion is subjected to the printing impact, the bonded portion is liable to be damaged earlier than the other portions. This tendency is more conspicuous as the printing speed increases. Accordingly, to respond to the demand for high speed printing in recent years, it has been demanded to reduce the damage to such a bonded portion as much as possible.